Laser optical apparatus

ABSTRACT

There is provided a structure for reducing optical loss in an optical apparatus (homogenizer) for making the intensity distribution of a laser beam uniform. 
     In a multi-cylindrical lens (a glass substrate having a multiplicity of cylindrical lenses formed thereon) used in a homogenizer, convex cylindrical lenses and concave cylindrical lenses are arranged alternately, and the boundaries between the cylindrical lenses have a smooth structure. This makes it possible to reduce scattering of beams that has occurred at the boundaries between the cylindrical lenses.

This is a continuation of U.S. application Ser. No. 09/041,152, filedMar. 9, 1998, (pending).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention disclosed in this specification relates to opticalapparatuses utilizing a laser such as apparatuses for performing anannealing process by means of irradiation with laser beams (laserannealing apparatuses) and, more particularly, to an laser annealingapparatus projecting beams with a large area which is capable ofproviding an uniform effect of irradiation. Such a laser annealingapparatus is used in semiconductor manufacturing steps and the like.

Laser beams with a large area are used in apparatuses including exposureapparatuses for forming fine circuit patterns such as semiconductorcircuits. Especially, ultraviolet laser beams are used for formingcircuits with design rules on a sub-micron basis.

2. Description of the Related Art

A techniques for crystallizing amorphous silicon films by irradiatingthem with laser beams has been known. Another known laser irradiationtechnique is to irradiate silicon films with laser beams in order torecover them from damage to crystallinity thereof due to theimplantation of impurity ions and in order to activate the implantedimpurity ions. Such techniques are referred to as “laser annealingtechniques”.

A typical example of the latter technique is the annealing of the sourceand drain regions of a thin film transistor. Those regions are annealedby irradiating them with laser beams after ions of impurities, typicallyphosphorus or boron, are implanted into those regions.

Such a process of irradiation with laser beams is characterized by thefact that there is substantially no thermal damage to a substrate.

The feature of giving no thermal damage to a substrate reduceslimitations on the materials to be subjected to such a process and isadvantageous, for example, in forming a semiconductor device on asubstrate made of glass or the like which has low heat resistance. Thisfeature is especially important in the fabrication of active matrixliquid crystal displays which recently have an increasing range ofapplication.

For an active matrix liquid crystal display, it is desirable to use aglass substrate from the viewpoint of cost and the requirement for alarger surface area.

A glass substrate can not withstand a heating process at temperatures ashigh as 600° C. or more or 700° C. or more. An effective technique foravoiding this problem is to perform annealing after the crystallizationof a silicon film and the implantation of impurity ions as describedabove by irradiating it with laser beams.

Even when a glass substrate is used, a method employing irradiation withlaser beams results in substantially no damage to the glass substrate.It is therefore possible to use a glass substrate in fabricating a thinfilm transistor having a crystalline silicon film.

There has been another proposal to use laser beams as a light source forforming fine circuit patterns taking advantage of the fact that laserbeams are coherent light. Especially, the use of an ultrasonic lasermakes it possible to obtain fine patterns having sizes in sub-microns orsmaller.

However, since laser beams have small beam areas when they are generatedby a laser apparatus (hereinafter they are referred to as “sourcebeams”), it is common to process a large surface area by scanning laserbeams across it. This results in problems including low uniformity ofthe effect of the process in a surface and a long period of timerequired for the process. Especially, common source beams result in asignificant problem from the viewpoint of uniformity of the effect ofprocessing when used as they are because they have non-uniformdistribution of light intensity.

Under such circumstances, a technique has been proposed wherein sourcebeams are processed into beams having highest possible uniformity andthe beam size is changed in accordance with the shape of the surfacearea to be processed and the like. Common beam shape is rectangular orlinear shape. Such an arrangement makes it possible to perform uniformlaser annealing over a large surface area.

FIG. 1A shows an example of a laser irradiation apparatus in whichsource beams are processed. For example, an excimer laser is used as thelaser oscillator. Laser beams are oscillated by decomposingpredetermined gases by means of RF discharge to produce an excited statereferred to as “excimer state”.

For example, in a KrF excimer laser, an excited state KrF* is obtainedby high voltage discharge using Kr and F as raw material gases. Whilethis excited state is unstable as indicated by its duration in the rangefrom several nano-seconds to several micro-seconds, KrF in the groundstate is more unstable. This results in inverted population wherein thedensity in the excited state is higher than the density in the groundstate. As a result, induced radiation occurs, which makes it possible toobtain laser beams having relatively high efficiency.

The laser oscillator is not limited to an excimer laser, and other pulselasers or continuous lasers may be used. In general, pulse lasers areappropriate for the purpose of achieving a high energy density.

As shown in FIG. 1A, a source beam emitted by the laser oscillator isprocessed into an appropriate size by a beam expander formed by aconcave lens or a convex lens.

The beam then enters an optical device referred to as “homogenizer”which includes at least one lens device (multi-cylindrical lens) havinga multiplicity of cylindrical lenses (generally in a parabolicconfiguration) . As shown in FIG. 1B, a conventional multi-cylindricallens includes a plurality of cylindrical lenses 1 through 5 (which areall convex lenses) formed on a single sheet of glass.

In general, two multi-cylindrical lenses are used and arranged so thatthey are perpendicular to each other. Obviously, the number of themulti-cylindrical lens may be one or three or more. When onemulti-cylindrical lens is used, the non-uniformity of a source beam inone direction is dispersed. When two or more multi-cylindrical lensesare formed in the same direction, the same effect as increasing thenumber of the cylindrical lenses can be achieved.

When a beam passes through the multi-cylindrical lens, the beam can beconverted into a uniform beam having a distributed energy density. Theprinciple behind this will be described later. Thereafter, the beam isprocessed by a converging lens into a desired shape or, if needed,deflected by a mirror to be projected upon a sample (see FIG. 1A).

A description will now be made on the principle of a conventionalhomogenizer (multi-cylindrical lens) and a problem of the same which isthe problem to be solved by the invention. In order to avoidcomplication, discussion on an optical basis will be focused on only onesurface. Laser beams that have passed through a multi-cylindrical lensare as shown in FIG. 2A.

Here, the multi-cylindrical lens L includes five convex cylindricallenses 1, and the beam incident upon each of the cylindrical lenses isrefracted by the cylindrical lens. After being converged at a focalpoint, the beams are diffused. This process results in a region in whichall of the beams that pass through the respective cylindrical lenses aremixed (mixed region).

Let us assume here that the distribution of the optical intensity of thebeams is polarized, resulting in differences in the intensity of thebeams incident upon the respective cylindrical lenses. In the mixedregion, however, such polarization is scattered because the beams thatpass through the respective cylindrical lenses are mixed. That is, theoptical intensity is made uniform. It is thus possible to obtain beamshaving less distribution of optical intensity (see FIG. 2A).

When we look at the paths of the beams that pass through themulti-cylindrical lens, the beams can be regarded as beams emitted frompoint light sources F (i.e., focal points) arranged at equal intervals(distances “a”) (see FIG. 2B).

The same effect can be achieved by providing a convex cylindrical lensl₁ on one side of a glass substrate and a convex cylindrical lens l₂ onthe other side at an interval “a”. In FIG. 3A, the path of a beam thathas passed through the cylindrical lens l₁ is indicated by the solidline, and the path of a beam that has passed through the cylindricallens l₂ is indicated by the broken line. In this case, a mixed region isobtained as in the case shown in FIG. 2A (see FIG. 3A).

When we look at the paths of the beams that pass through themulti-cylindrical lens, as shown in FIG. 3B, the beams can be regardedas beams emitted from two kinds of point light sources F₁ and F₂ (i.e.,focal points) (see FIG. 3B).

In the case of the conventional multi-cylindrical lens as describedabove, since there is an angle at the ends of each cylindrical lens(boundaries between itself and other cylindrical lenses), beams havebeen scattered at such regions, which has resulted in optical loss. Thismeans that the laser beams can not be effectively utilized and the beamintensity is reduced.

SUMMARY OF THE INVENTION

The present invention has been conceived taking the above-describedproblem into consideration. A multi-cylindrical lens according to thepresent invention is characterized in that not only convex cylindricallenses but also concave cylindrical lenses are used; the convexcylindrical lenses and concave cylindrical lenses are alternatelyarranged; and the cylindrical lenses smoothly continue to each other.

According to a first aspect of the invention, a multi-cylindrical lenshaving the above-described configuration is provided in a homogenizerwhich is inserted and used between a laser oscillator and an object tobe irradiated in an apparatus for forming a laser beam having a linearor rectangular beam shape.

According to a second aspect of the invention, in a laser opticalapparatus having a laser oscillator and a homogenizer to which a laserbeam emitted by the laser oscillator is input, a multi-cylindrical lensused in the homogenizer have the above-described configuration.

In the above-described multi-cylindrical lens, the state of the curvedsurface of the convex cylindrical lenses may be the same as the state ofthe curved surface of the concave cylindrical lenses. Further, at leasttwo multi-cylindrical lenses may be provided in the homogenizer, and thetwo multi-cylindrical lenses may be provided in directions perpendicularto each other.

The configuration of a multi-cylindrical lens according to the inventionis as shown in FIG. 1C. Specifically, while any conventionalmulti-cylindrical lens has been formed by convex cylindrical lenses 1through 5 (see FIG. 1B), concave cylindrical lenses 2 and 4 are providedbetween the convex cylindrical lenses 1 and 3 and between the convexcylindrical lenses 3 and 5, respectively, and the boundaries betweenthose cylindrical lenses are smooth and continuous (see FIG. 1C).

The cylindrical lenses can be smoothly connected to each other by makingthe curvature (the shape of the curved surface) of a concave cylindricallens identical to a convex cylindrical lens adjacent thereto. When thecylindrical lenses are parabolic lenses, there is provided a structurewhich is a combination of parabolic surfaces having differentorientations. The paths of laser beams in such a multi-cylindrical lensare as shown in FIGS. 4A and 4B when illustrated in the same manner asin FIGS. 2A and 2B and FIGS. 3A and 3B.

Here, the multi-cylindrical lens L includes three convex cylindricallenses l and two concave cylindrical lenses 12 as shown in FIG. 1C.Beams incident upon the convex cylindrical lenses are diffused afterbeing converged at focal points. On the other hand, beams incident uponthe concave cylindrical lenses are simply diffused as if they arediffused from certain points. As a result, there is obtained a regionwhere all of the beams that pass through the cylindrical lenses aremixed (mixed region).

The above-described effect can be similarly achieved even when theconvex cylindrical lenses and the concave cylindrical lenses havedifferent curvatures (a factor that determines a focal distance or theshape of the curved surface of a lens). In addition, optical loss can bereduced in a multi-cylindrical lens having such a configuration becausethere is no structure that scatters beams (an unsmooth region such as aprotrusion) (see FIG. 4A).

When we look at the paths of the beams that pass through themulti-cylindrical lens, the beams can be regarded as beams emitted fromtwo kinds of point light sources F₁ and F₂ (i.e., focal points) as thoseshown in FIG. 3B (see FIG. 4B).

When the convex cylindrical lenses and the concave cylindrical lenseshave the same curvature, the boundaries of the paths of the beams thatpass through the concave cylindrical lenses pass through the focalpoints F₁ of the adjacent convex cylindrical lenses. A descriptionfollows on this point. When convex cylindrical lenses and the concavecylindrical lenses have the same curvature, the angle of convergence bythe former (the angle at which the beams are diffused after passingthrough the focal points) is the same as the angle of diffusion by thelatter when collimated beams are incident.

Specifically, when the focal distance of a convex cylindrical lens l₁ isrepresented by x as shown in FIG. 5A, collimated beams that pass througha concave cylindrical lens l₂ can be regarded as beams emitted from apoint F₂ which is at the distance x toward the side at which the beamsenter. Let us consider the path F₁A of a beam that passes through thelower end of the convex cylindrical lens l₁ and the path F₂A of a beamthat passes through the upper end of the concave cylindrical lens l₂.Then, since the angles of diffusion and convergence of the beams thatpass through those lenses are the same, the line F₂A overlaps the lineF₁A. That is, the boundary of the path of the beam passing through theconcave cylindrical lens passes through the focal point F₁ of the convexcylindrical lens adjacent thereto (FIG. 5B).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic view of an optical system of a laser irradiationapparatus.

FIG. 1B is a schematic view of a conventional multi-cylindrical lens.

FIG. 1C is a schematic view of a multi-cylindrical lens according to thepresent invention.

FIGS. 2A and 2B are schematic views showing beam paths in a conventionalmulti-cylindrical lens.

FIGS. 3A and 3B are schematic views showing beam paths in a conventionalmulti-cylindrical lens.

FIGS. 4A and 4B are schematic views showing beam paths in amulti-cylindrical lens according to the present invention.

FIGS. 5A and 5B are views showing beam paths of convex lenses andconcave lenses.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

An optical system of an embodiment of the present invention will now bedescribed. A laser irradiation apparatus according to the presentembodiment has the same basic configuration as that shown in FIG. 1A.The shape of a laser beam before incidence upon a homogenizer isexpressed by 6 cm×5 cm. In this embodiment, a multi-cylindrical lens isused as the homogenizer. Here, only the multi-cylindrical lens will bedescribed.

In the configuration shown in this embodiment, the multi-cylindricallens is formed by arranging six concave cylindrical lenses (having awidth of 5 mm) and five convex cylindrical lenses (having a width of 5mm) alternately to divide an incident beam into about ten beams. Thelength of the cylindrical lenses in the longitudinal direction thereofis 7 cm. The multi-cylindrical lens is made of quartz.

In the present embodiment, the length of a liner laser beam that isfinally projected is 12 cm in the longitudinal direction thereof and thewidth is 0.5 mm. As a result, the laser beam is enlarged by a factor of2 in the longitudinal direction and is reduced by a factor of 100 in thedirection perpendicular thereto after the laser beam passes through thehomogenizer. All of the convex cylindrical lenses and concavecylindrical lenses are spherical lenses and have the same curvature. Thefocal distance of the convex cylindrical lenses is 5 cm (see FIG. 1A).

The use of the invention disclosed in this specification makes itpossible to obtain a uniform laser beam having a large area required ina laser process used for the fabrication of a semiconductor device andthe like.

It should be understood that the foregoing description is onlyillustrative of the invention. Various alternatives and modificationscan be devised by those skilled in the art without departing from theinvention. Accordingly, the present invention is intended to embrace allsuch alternatives, modifications and variances which fall within thescope of the appended claims.

What is claimed is:
 1. A laser optical apparatus for forming a laserbeam comprising: a homogenizer located between a laser oscillator and anobject to be irradiated, said homogenizer comprising at least onemulti-cylindrical lens; and at least one converging lens located betweensaid homogenizer and said object to be irradiated, wherein said at leastone multi-cylindrical lens comprises a plurality of convex cylindricallenses and concave cylindrical lenses arranged alternately, and a convexsurface of one of said convex cylindrical lenses and a concave surfaceof one of said concave cylindrical lenses adjacent to each other aresmoothly continuous without having an angle therebetween.
 2. The laseroptical apparatus according to claim 1, wherein said laser beam has alinear beam shape on said object to be irradiated.
 3. The laser opticalapparatus according to claim 1, wherein said laser beam has arectangular beam shape on said object to be irradiated.
 4. A laseroptical apparatus for forming a laser beam comprising: a homogenizerlocated between a laser oscillator and an object to be irradiated, saidhomogenizer comprising two multi-cylindrical lenses; and at least oneconverging lens located between said homogenizer and said object to beirradiated, wherein each of two multi-cylindrical lenses comprises aplurality of convex cylindrical lenses and concave cylindrical lensesarranged alternately, a convex surface of one of said convex cylindricallenses and a concave surface of one of said concave cylindrical lensesadjacent to each other are smoothly continuous without having an angletherebetween, and said two multi-cylindrical lenses are arranged indirections perpendicular to each other.
 5. The laser optical apparatusaccording to claim 4, wherein said laser beam has a linear beam shape onsaid object to be irradiated.
 6. The laser optical apparatus accordingto claim 4, wherein said laser beam has a rectangular beam shape on saidobject to be irradiated.
 7. A laser optical apparatus comprising: alaser oscillator for producing a laser beam; a first multi-cylindricallens comprising a plurality of first convex lenses and first concavelenses arranged alternately wherein said first multi-cylindrical lenscondenses and diverges said laser beam from said laser oscillator onlyin a first direction; a second multi-cylindrical lens comprising aplurality of second convex lenses and second concave lenses arrangedalternately wherein said second multi-cylindrical lens condenses anddiverges the laser beam passing through the first multi-cylindrical lensonly in a second direction orthogonal to said first direction; and aconverging lens for condensing the laser beam passing through the secondmulti-cylindrical lens.
 8. The laser optical apparatus according toclaim 7, wherein said laser beam has a linear beam shape on an object tobe irradiated.
 9. The laser optical apparatus according to claim 7,wherein said laser beam has a rectangular beam shape on an object to beirradiated.
 10. A laser optical apparatus comprising: a laser oscillatorfor producing a laser beam; a first multi-cylindrical lens comprising aplurality of first convex lenses and first concave lenses arrangedalternately wherein said first multi-cylindrical lens condenses anddiverges said laser beam from said laser oscillator only in a firstdirection; a second multi-cylindrical lens comprising a plurality ofsecond convex lenses and second concave lenses arranged alternatelywherein said second multi-cylindrical lens condenses and diverges thelaser beam passing through the first multi-cylindrical lens only in asecond direction orthogonal to said first direction; a first converginglens for condensing the laser beam passing through the firstmulti-cylindrical lens; and a second converging lens for condensing thelaser beam passing through the second multi-cylindrical lens.
 11. Thelaser optical apparatus according to claim 10, wherein said laser beamhas a linear beam shape on an object to be irradiated.
 12. The laseroptical apparatus according to claim 10, wherein said laser beam has arectangular beam shape on an object to be irradiated.
 13. A laseroptical apparatus comprising: a laser oscillator for producing a laserbeam; a beam expander located between said laser oscillator and anobject to be irradiated; a homogenizer located between said beamexpander and said object to be irradiated, said homogenizer comprisingat least one multi-cylindrical lens; and at least one converging lenslocated between said homogenizer and said object to be irradiated,wherein said at least one multi-cylindrical lens comprises a pluralityof convex cylindrical lenses and concave cylindrical lenses arrangedalternately, and a convex surface of one of said convex cylindricallenses and a concave surface of one of said concave cylindrical lensesadjacent to each other are smoothly continuous without having an angletherebetween.
 14. The laser optical apparatus according to claim 13,wherein said laser beam has a linear beam shape on said object to beirradiated.
 15. The laser optical apparatus according to claim 13,wherein said laser beam has a rectangular beam shape on said object tobe irradiated.
 16. A laser optical apparatus comprising: a laseroscillator for producing a laser beam; a beam expander located betweensaid laser oscillator and an object to be irradiated; a homogenizerlocated between said beam expander and said object to be irradiated,said homogenizer comprising two multi-cylindrical lenses; and at leastone converging lens located between said homogenizer and said object tobe irradiated, wherein each of two multi-cylindrical lenses comprises aplurality of convex cylindrical lenses and concave cylindrical lensesarranged alternately, a convex surface of one of said convex cylindricallenses and a concave surface of one of said concave cylindrical lensesadjacent to each other are smoothly continuous without having an angletherebetween, and said two multi-cylindrical lenses are arranged indirections perpendicular to each other.
 17. The laser optical apparatusaccording to claim 16, wherein said laser beam has a linear beam shapeon said object to be irradiated.
 18. The laser optical apparatusaccording to claim 16, wherein said laser beam has a rectangular beamshape on said object to be irradiated.
 19. A laser optical apparatuscomprising: a laser oscillator for producing a laser beam; a beamexpander for expanding said laser beam; a first multi-cylindrical lenscomprising a plurality of first convex lenses and first concave lensesarranged alternately wherein said first multi-cylindrical lens condensesand diverges said laser beam from said laser oscillator only in a firstdirection; a second multi-cylindrical lens comprising a plurality ofsecond convex lenses and second concave lenses arranged alternatelywherein said second multi-cylindrical lens condenses and diverges thelaser beam passing through the first multi-cylindrical lens only in asecond direction orthogonal to said first direction; and a converginglens for condensing the laser beam passing through the secondmulti-cylindrical lens.
 20. The laser optical apparatus according toclaim 19, wherein said laser beam has a linear beam shape on an objectto be irradiated.
 21. The laser optical apparatus according to claim 19,wherein said laser beam has a rectangular beam shape on an object to beirradiated.
 22. A laser optical apparatus comprising: a laser oscillatorfor producing a laser beam; a beam expander for expanding said laserbeam; a first multi-cylindrical lens comprising a plurality of firstconvex lenses and first concave lenses arranged alternately wherein saidfirst multi-cylindrical lens condenses and diverges said laser beam fromsaid laser oscillator only in a first direction; a secondmulti-cylindrical lens comprising a plurality of second convex lensesand second concave lenses arranged alternately wherein said secondmulti-cylindrical lens condenses and diverges the laser beam passingthrough the first multi-cylindrical lens only in a second directionorthogonal to said first direction; a first converging lens forcondensing the laser beam passing through the first multi-cylindricallens; and a second converging lens for condensing the laser beam passingthrough the second multi-cylindrical lens.
 23. The laser opticalapparatus according to claim 22, wherein said laser beam has a linearbeam shape on an object to be irradiated.
 24. The laser opticalapparatus according to claim 22, wherein said laser beam has arectangular beam shape on an object to be irradiated.
 25. A laseroptical apparatus comprising: a laser oscillator for producing a laserbeam; and a homogenizer located between said laser oscillator and anobject to be irradiated, said homogenizer comprising at least onemulti-cylindrical lens, wherein said at least one multi-cylindrical lenscomprises a plurality of first convex cylindrical lenses and secondconvex cylindrical lenses arranged alternately, each of said firstconvex cylindrical lenses has a curved surface which said laser beamenters, and each of said second convex cylindrical lenses has a planesurface which said laser beam enters.
 26. The laser optical apparatusaccording to claim 25, wherein said laser beam has a linear beam shapeon said object to be irradiated.
 27. The laser optical apparatusaccording to claim 25, wherein said laser beam has a rectangular beamshape on said object to be irradiated.